The production of carboxylic acids by the carbonylation of alcohols with carbon monoxide is well known. One example, the carbonylation of methanol, is the most important reaction for the commercial production of acetic acid. Acetic acid, which is a principal ingredient in vinegar, has hundreds of uses in addition to giving flavor to cooking and salads. Paints and adhesives contain latex emulsion resins polymerized from vinyl acetate monomer (VAM) made with acetic acid, and some cellulosic fibers and plastics are manufactured from acetic anhydride derived from acetic acid.
Carbonylation of methanol to acetic acid is an exothermic reaction. The heat of reaction must be removed in order to control the temperature in the carbonylation reaction zone. The methods by which the prior art processes remove this heat of reaction are exemplified by the teachings of U.S. Pat. Nos. 5,334,755 (Yoneda et al.) and 5,364,963 (Minami et al.). One method, which is used with a continuous flow stirred tank type reactor, comprises feeding a portion of the solution in the reactor to a cooler, which cools that portion of the solution by indirect heat exchange with a cooling medium. The cooled solution is then recycled to the reactor. Another method, which is used with a plug flow type reactor that has a plurality of catalyst-containing pipes, comprises passing the reaction solution through the pipes while passing a cooling medium through the sheath that surrounds the pipes. For this method, the cooling medium is saturated boiler feed water which, after having been heated and vaporized to low pressure steam, may be used as a heat source for a distillation tower that recovers the acetic acid product. Yet another method, which is also used with a continuous flow stirred tank type reactor, comprises installing a cooling coil inside the reactor and passing a cooling medium through the coil.
One of the problems with both of these prior art methods is that the heat of reaction is transferred in two steps, first to a cooling medium, and then from the now-heated cooling medium to some other heat sink. In some prior art processes, the cooling medium cannot transfer any heat to the other heat sink, because the temperature of the cooling medium is less than or equal to the temperature of the other heat sink. In other prior art processes, the temperature of the cooling medium is greater than the temperature of the other heat sink so that in theory heat could be transferred from the cooling medium to the other heat sink. However, in practice heat cannot be transferred because the difference in the temperatures of the cooling medium and the other heat sink is so small that the surface area required to accomplish the heat transfer would be impracticably large and the necessary heat transfer equipment would be prohibitively expensive. Accordingly, even if the other heat sink is a stream within the carbonylation process, this two-step transfer of heat is inefficient because a significant proportion of the heat of reaction in the prior art methods is poorly or inefficiently utilized.
The inefficiency of the prior art methods, which both use a cooling medium, arises in both heat transfer steps. In the first step, the temperature of the cooling medium cannot as a practical matter be heated to the maximum temperature of the reaction solution. Rather, the cooling medium is heated only to a temperature that is less than the maximum temperature of the reaction solution by a temperature difference that is equal to the hot end approach of the heat exchanger that is employed in the first step. The heat of reaction that corresponds to the temperature difference of the hot end approach is left unused and is effectively lost, because it is never transferred to the cooling medium.
Similarly, in the second step, the temperature of the cooling medium cannot as a practical matter be cooled to the minimum temperature of the other heat sink. Instead, the cooling medium is cooled only to a temperature that is greater than the minimum temperature of the heat sink by a temperature difference equal to the cold end approach of the heat exchanger that is employed in the second step. Accordingly, the heat of reaction that corresponds to the temperature difference of the cold end approach is left unused and is also effectively lost, because it is never transferred from the cooling medium.
Taking into account both heat transfer steps, the total unutilized heat of reaction is thus represented by the sum of the hot end approach of the first step and the cold end approach of the second step. If the heat of reaction and/or the difference between the maximum temperature of the reaction zone and the minimum temperature of the heat sink is relatively small, this total unutilized heat of reaction can be a large percentage of the total heat of reaction.
Accordingly, methods of utilizing the heat of reaction in a carbonylation process are sought that use the heat of reaction in a manner that is more efficient than the prior art methods.